Control strategies for internal combustion engines utilising fuel injection, such as compression ignition or spark-ignition internal combustion engines, are known in which the timing of fuel injections and the quantity of fuel delivered by each injection is varied in accordance with engine operating conditions. In the case of an internal combustion engine utilising direct fuel injection (DFI), such as a gasoline direct injection (GDI) internal combustion engine, in which fuel is injected directly into the engine cylinders, it is known at ‘key on’ start, and also for onward driving, to inject fuel according to a fuel injection pattern or strategy which maximises fuel efficiency and minimises emissions of hydrocarbons and soot.
For example, an optimum fuel injection pattern may involve splitting the required fuel into two or three separate injections and delivering these injections into a cylinder during the intake stroke of its respective piston. FIG. 1 is a bar chart showing relative quantities of engine emissions of hydrocarbons and soot resulting from different fuel injection patterns of the prior art. This ‘split intake’ injection pattern achieves a relatively low level of emissions, as indicated by bar 10 in FIG. 1. This is because the intake stroke is early in the piston cycle so that the injected fuel has time to vaporise before a spark ignites the air/fuel mixture. The splitting of the injections also promotes fuel vaporisation because each one of the smaller quantity injections of fuel can readily vaporise before the next is delivered. Furthermore, good fuel preparation is facilitated by virtue of injection occurring during the turbulent induction phase when the piston is disposed sufficiently far down the cylinder so as to avoid direct fuel to piston impingement, and the fuel has to travel a large distance through turbulent air, which promotes good mixing. This relatively low level of emissions will be appreciated from a comparison of bar 10 with bar 12 of FIG. 1 which represents the higher emissions resulting from a single injection of fuel during the intake stroke.
The amount of emissions resulting from the use of a “split” fuel injection pattern can be reduced further by modulating the amount of air let into each cylinder using a continuously variable valve lift (CVVL). A ‘low lift’ arrangement of the valve creates a rapid flow of air into the cylinder leading to turbulence which promotes homogenous mixing of the fuel and air inside the cylinder for a more efficient combustion. The reduced emissions resulting from this arrangement are indicated by bar 14 in FIG. 1.
FIG. 2A is a schematic illustration of a split intake injection pattern of the prior art. Referring to FIG. 2A, a full revolution of an engine crankshaft is represented by a circle 20, starting from an uppermost position of a particular engine piston (the top of the circle 20) and moving through intake and compression strokes (clockwise around the circle 20), back to the uppermost position. In FIG. 2A, the relative positions of four pistons, labelled A, B, C and D are shown, each piston position being 90° apart, respectively. The right hand side of the circle 20 represents the intake stroke and the left hand side represents the compression stroke. Thus, when cylinder D is in the induction stroke, cylinder A is in the compression stroke prior to “sparking” at top dead centre (TDC). As shown, fuel is delivered in first, second and third injections 22, 24 and 26, during the intake stroke of the piston.
Although a split intake injection pattern generates low emissions, there is a disadvantage to using such an injection strategy when starting the engine, for example during an eco-start when rapid starting of the engine is required. By eco start is meant the restarting of a vehicle engine after the vehicle has automatically stopped the engine during the course of a drivecycle to save fuel when conditions permit. For example, a vehicle may be arranged to adopt an eco stop condition when a driver-operated brake pedal of the vehicle is depressed and the vehicle is stationary. When the driver releases the brake pedal the engine may be restarted and a transmission of the vehicle may be re-engaged. That is, release of the brake pedal by the driver triggers the engine to be restarted, the driveline to be closed and torque to be transmitted to the drive wheels. However, restarting the engine from an eco stop condition should not cause nuisance to the driver and there should be no appreciable delay in the reapplication of torque to the drive wheels. As a result, there is a requirement for an engine to be started rapidly during eco start. In the context of the present application, an eco-stop/start functionality may also be provided when a vehicle is moving, for example, in a hybrid vehicle in which the internal combustion engine may be stopped and re-started as required when the vehicle is not stationary.
To provide a rapid start, it is known to instead apply a compression injection pattern during a start-up phase of the engine. FIG. 2B is a schematic illustration of a compression injection pattern of the prior art. As shown in FIG. 2B, a single injection of fuel 28 may be provided late in the compression stroke to each engine cylinder, shortly before an ignition spark. Although this strategy provides the rapid start required for an eco-start, the start-up phase itself is associated with high emissions as it relies on a compression injection pattern. The high emissions associated with a compression injection pattern are illustrated by bar 16 in FIG. 1. This can be readily compared with the lower emissions associated with single intake, split intake and split low lift intake injection patterns as indicated by the bars 12, 10 and 14, respectively. The increased emissions associated with the compression injection pattern are a consequence of the necessity of the late timing required to combust the cylinder in the compression stroke in order to meet the desired short eco-start time. It is an object of the present invention to substantially overcome and/or mitigate at least some of the problems described above.